Controlling the isoelectric point of gelatin



April 9, 1946.

R. C. HOUCK ET AL CONTROLLiNG THE ISOELEC'I'RIC POINT OF GELATIN FiledJuly 20, 1943 I I m l I EFFECT OF TIME OF REACTION OF ALKAL/ ON THEISOELECTH'IC POINT OF GELATIN P IZZ I Z 3 4 I 5 6 7 8 TIME (IN HOURS)'AT 52C. 55mm? ADDING A'clo ROBERT c. HOUCK JOHN RUSSELL SAMUEL E.SHEPPARD INVENTORS BY W. M

(c? sawzJmM ATTORNEYS tracted by the acid process.

Patented Apr. 9, 1946 CONTROLLING THE ISOELECTRIC POINT OF GELATINRobert C. Houok, John Russell, and Samuel E.

Sheppard, Rochester, N. Y., assignors to Eastman Kodak Company,Rochester, N. Y., a corporation of New Jersey Application July 20, 1943,Serial No. 495,454

6 Claims.

This invention relates to the manufacture of gelatin and particularly toa method for controlling the isoelectric point of gelatin.

It is known that the properties of gelatin can be changed markedly bythe method of manugelatin, the isoelectric point is thepH at which it isequally dissociated as an acid and as a base. The isoelectric point ofgelatin depends upon the method by which the gelatin has been made.Gelatins made by the lime process of plumping of hides have anisoelectric point of from 4.7 to 5.0 and their physical propertiesusually exhibit a sharp minimum somewhere in.this region. Gelatinsextracted by the acid process have an isoelectric point varying from pH7 to pH 9 and their physical properties usually do not exhibit a sharpminimum but a flat region as shown, for example, in Sheppard and HudsonU. S. Patent 2,101,877, granted December 14, 1937.

It is desirable in certain processes to have a gelatin having anisoelectric point below the values usually obtained when the material isex- The pH of the isoelectric point of the .gelatin is not readilycontrolled by varying the method of extraction from the hides and thegelatin obtained does not always have an isoelectric point in thedesired region.

It is, therefore, an object of the present invention to provide a methodfor controlling theisoelectric point of gelatin. A further object is toprovide a method for obtaining gelatin of any desired isoelectric point.A still further object is to provide a method for lowering theisoelectric point of gelatin and thereby controlling its physicalproperties such as swelling and viscosity. Other objects will appearfrom the following description of our invention.

These objects are accomplished by heating gelatin in the presence of anymaterial which raises the pH of the gelatin solution, such as analkaline hydroxide, for a definite period of time and then stopping theaction of the alkali and washing it from the gelatin.

The accompanying drawing shows a series of graphs illustrating thechange in isoelectric point obtained by treating an acid process gelatinfor varying periods of time at various degrees of alkalinity.

'Our method consists in treating gelatin, preferably gelatin of highisoelectric point, with any material which raises the pH of thesolution, such as an alkali metal hydroxide, an alkaline earth metalhydroxide, an alkali carbonate, triethanolamine, or pyridine for adefinite period of time. The time required depends upon the temperatureand the pH at which the gelatin is heated or aged. The speed of the dropin isoelectric point at a given pHis affected by the temperature whichcan be varied from 5 C. to C. The pH can be varied from 8 to 12.5, therate change at a given temperature increasing with increase in pH from 8to 12.5. At the end of the desired time, the action of the alkali isstopped by adding acid to neutralize the alkali. The resulting gelatinis chilled, noodled and washed. It will then have an isoelectric pointwhich depends upon the time, temperature and pH of treatment as statedabove.

According to our method a solution of gelatin is made in the usualmanner and the desired amount of an aqueous solution of analkalinematerial, such as alkali metal hydroxide or alkaline earth metalhydroxide added to it to bring the pH to the desired point as explainedbelow. The solution is then heated for a definite period of time and atthe end of this time the reaction is stopped by adding suflicient acidsuch as hydrochloric acid to neutralize the alkali which was added tothe gelatin. The resulting gelatin is then chilled, noodled and Washedfor about 48 hours with colddistilled water.

We have treated both acid-process gelatin and lime-process gelatin byour method but find it preferable to treat the acid-process gelatin. Re-

sults obtained by treating the various types of mined and are given inthe following tables.

The effect of pH on the isoelectric point of an acid process gelatintreated at 52 C. for various lengths of time is recorded in thefollowing table. I

to 8 and the higher values at which portions of" the gelatin weretreated. The results of this treatment were as follows:

pH of isoelec'tricpoint Time of treatment (hours) pH 9.6 pH 10.6

These data were plotted as shown in the accompanying drawing'to-indic'atethe change in DHlof the isoelectric point for various timesof treatment. A study of the table and the curves shows thatthe decreasein'isoelectric point is regulated at a giv'en'temperature by the pH ofthe solution and the time of heating. As shown, for exampleyin the curvefor treatment of the gelatin at pH 10.6, the isoelectric point wasreduced after two hours treatment at 52 C. from 9.17 to 7.04. After fivehours treatment the iso-, electric point of this sample had dropped to6.46 and, after 28 hours treatment it had dropped to 5.4. As the pH oftreatment increased, the rate of decrease in isoelectric point showed anincrease; For example, at a, pH of treatment of 12.2 the isoelectricpoint dropped to 5.15 after one hour whereas at a pH of treatment of10.6 it had dropped only to 7.64 after the same length of time. v I

' In order to illustrate the effect of temperature on the lowering ofthe isoelectric point of acidprocess gelatin, a sampled the same gelatinhaving an original isoelectricpoint at pH 9.17 was treated with asufficient quantity of sodium hydroxide solution to bringthe pH of thegelatin to 9.6. At this pH the gelatin was treated at temperatures offrom 35 C. to 52 C. and the results recorded as follows:

- Time of PH Temperature a electric heatin point r 7 Hours 35 C 96 6. 52

C i 72 5. 77 7 52 C O 4. 91

point is about this pH. In spite of these difierences in timesof'heating the results indicate that the reaction is slowed up bylowering the temperature as was to be expected.

In order to study the eiiect of variation in the alkali employed, wehave used alkali metal hydroxides such as sodiumflhydroxide, {potassiumhydroxide and ammonium hydroxide and alkaw .line earth metal hydroxidessuch as calcium hy- 'method employed to raise the pH does not afiectdroxide. Results of these tests show that the the result appreciably. v

The swelling of gelatin is one physical property whose minimum rangeincludes the isoelectrlc point.-

ing as compared to lime-process gelatin with a sharp minimum at theisoelectric point, Swelling measurements were made on gelatin having anisoelectric point of 6.5 after treatment of-an original gelatin havingan-isoelectric point'at'pI-I 8.4. Swelling measurements were made at 15C. using a sodium acetate-acetic acid buffer andthe regionof minimumswelling was'found to extend from pH 4.5to pH 10. This indicated thatthe gelatin had a region of minimum swelling in-the same region as thatof the isoelectric point. In using our method for the treatment oflimeprocess gelatin, we employed a deashedli'me process gelatin havinganfis'oelectric point of 4.78. On heating a sample of this'gelatin at 52C. for

' five hoursat pH'12. 2,fthe isoelectrie point' d'ecreased slightly frompH 4.78 to ;pH'4. 5. The gelatin thus obtained had been seriouslydegraded by this treatment'which was rather harsh. Aging at pH 12.2'forone monthat 5 C. did not decrease the isoelectric point. Under similarconditions the isoelectric point of acid-processgelatin would have beenmarkedly reduced. J

This result with lime process gelatin, differing from the resultobtained with'acid process gelatin is explained by the fact that thehide has lost most of its amide nitrogen during the liming process,Consequently the resulting gelatin has a relatively low isoelectricpoint andis' not atfected appreciably by further alkaline treatment. Theuse of gelatine of a definite isoelectric point isof value in thosecases where standardization of properties, such as swelling, isconcerned. In the use of only gelatine of a specific isoelectric point,all external conditions must be regulated by the isoelectric point ofthe gelatin. If gelatins of all isoelectric points are obtainable thenthe most desirable conditions with respect j '1. The method of reducingthe isoelectric point of gelatin havingan isoelectric point above pH 5',

which comprises treating said gelatin at a temperature of from about 30C. to about 60 C. for a period of time from about 1 minute to about 50hours with an alkaline hydroxide at a pH greater I than about 8, thenneutralizing the alkali and washing the gelatin, thereby effecting areduction in the isoelectric point of the gelatin.

2. The method of reducing-the isoelectric point 'of-gelatin having anisoelectric point above pH- 5, which comprises treating said gelatin ata temperature of from about 30 C. to about 60 C. for a period of timefrom about 1 minute to about 50 hours with an alkali metal hydroxide ata pH greater. than about 8, then neutralizing the alkali and washing thegelatin, thereby effecting a reduction in the isoelectric point of thegelatin.

3. The method of reducing the isoelectric point of acid-process gelatin,which comprises adding sodium hydroxide to a solution of said gelatin insuificient amount to bring the pH to about 8, heating the mixture at atemperature of from about 30 C. to about 60 C. for a period of time fromabout one minute to about 50 hours, neutralizing the alkali with acid,and washing the gelatin with cold water.

4. The method of reducing the isoelectric point of acid-process gelatin,which comprises adding sodium hydroxide to a solution of said gelatin insufiicient amount to bring the pH to about 8, heating the mixture at atemperature of from about 30 C. to about 60 C. for a period oftime fromabout 1 minute to about 50 hours, neutralizing the alkali withhydrochloric acid, and washing the gelatin with cold water.

5. The method of treating gelatin having an isoelectric point above pH5, to lower the pH of its isoelectric point, which comprises mixing analkali metal hydroxide with a solution of gelatin.

at a pH above about 8, heating the mixture at a temperature ofapproximately 50 C. for a period of time from about one minute to about50 hours, neutralizing the alkali metal hydroxide with acid, and washingthe gelatin with water.

6. The method of reducing the isoelectric point of gelatin having anisoeleotric point above pH 5, which comprises treating said gelatin withan alkaline material at a temperature of from about 5 C. to about 60 C.at a pH greater than about pH 8 and for a length 01 time sufficient toefiect a reduction in the isoelectric point of the gelatin withoutappreciably degrading it, then neutralizing the alkaline material andwashing the gelatin, thereby effecting a reduction in the isoelectricpoint of the gelatin,

ROBERT C. HOUCK. JOHN RUSSELL. SAMUEL E. SHEPPARD.

